Flatpak lets applications share a common set of libraries, called a runtime. So org.gnome.Platform has all the libraries like glib and gtk+ that pretty much all GNOME applications use, and all that's left to build are the ones that the application bundles that aren't in the runtime.

As audidude mentioned, but to add to it a little, is basically we have an SDK now. So you download (with delta update etc and sharing Sdk code) what's needed instead of compiling etc.